An Australian zoologist, Dr Andrew Pask, has found that by rubbing oestrogen on the tip of a human penis, a thickened layer of skin keratin can be formed that might help prevent the spread of HIV. The thinking behind this is that skin keratin helps prevent viral infections.
The epithelium of the human penis is richly supplied with oestrogen receptors suggesting it could respond to topical oestrogen.
Furthermore, according to the article:
To confirm its effect, topical oestrogen was applied to the human
foreskin for a two week trial. This resulted in a rapid and substantial
increase in keratin thickness.
This begs the question, though–does it affect circumcimsed men differently from uncircumsized men?